I have Cubase AI 12 from my recent Audio Interface.
I’ve been offered the Cubase Pro 10.5 Upgrade From Cubase AI, but I suppose AI 10 was out at that point.
So will the license system recognise AI 12 as a valid base for the Upgrade?
I’m assuming that I will get a grace period Upgrade from Pro 10.5 to 12 when I register; if it works with AI 12.
Now that Steinberg are using their own licensing system, will the grace period still work for such an old license?
Complicated! 
You can’t get 10.5. You can only upgrade to 12 pro. You can never go backwards.
Thanks but it shows as £411 to upgrade from AI 12.
Yes, @valleyken has probably mistaken the price, don’t expect it to drop by a crazy amount when upgrading from AI.
You already have the upgrade code, but not from the right version. When you activate a new licence, it always gives the current version, but the upgrade you have is probably “from Cubase 10.5”, so you won’t be able to use it on a Cubase 12 license. Anyway, try redeeming it and see if that works. If it doesn’t, you may need to contact the Steinberg Support so that they exchange the code with a valid one.
Indeed. I had mistaken Artist 12 for AI 12. It is more than a bit confusing with all those different versions.
I had Artist with my audio interface, but I guess they only bundle that with the audio interfaces that are already expensive by itself, like my UR-RT4.
Good to know is that only the initial upgrade to Pro is more expensive. Then it is usually a $99 upgrade to each new Pro version every year.
Remember AI comes free with audio interfaces. There is no way to buy it on its own. That’s why the upgrade to pro is so much as it’s given away.